You can set a meeting policy so anyone can join without the leader, see Automatically admit users at https://docs.microsoft.com/en-us/microsoftteams/meeting-policies-in-teams.

 

It does create some concern, for example if you are providing toll-free access at your cost then people could use your conference ID to avoid call charges, but if you have audio conferencing on a per user basis with toll lines there isn't a commercial risk,

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